1. Field of the Invention
The present invention relates to power converters and, in particular, refers to a linear voltage regulator.
2. Description of the Related Art
A linear voltage regulator accepts a poorly specified and sometimes fluctuating input voltage and provides a substantially constant output voltage at a desirable level. The output voltage is used as a supply voltage for other circuits and is substantially independent of an output current (i.e., a load current). In one aspect, the load current level varies over time with substantially instantaneous transitions from one level to another level. For example, the linear voltage regulator supplies power to one or more digital circuits within a computer system which can be on or off depending on processing requirements. Thus, the load current level can be relatively high in one clock cycle and relatively low in a following clock cycle. As the digital circuits continue to improve and operate at higher frequencies, the transitions between clock cycles become faster thereby decreasing the transition time between load current levels.
One example of a linear voltage regulator is a low-dropout (LDO) regulator, wherein the LDO regulator is characterized by its ability to regulate the output voltage at a low voltage differential across an input terminal and an output terminal of the LDO regulator. A pass element (e.g., a power transistor) connects in series between the input terminal and the output terminal of the LDO regulator, wherein the power transistor provides the load current to the output terminal of the LDO regulator. The power transistor is generally large. In high-speed applications, the LDO regulator typically operates at a high quiescent current to drive the power transistor at a reasonable speed, but the result of utilizing a high quiescent current is inefficient power regulation.
The present invention solves these and other problems by providing a linear voltage regulator with adaptive biasing. The linear voltage regulator (e.g., a LDO regulator), accepts an input voltage at an input terminal and provides a substantially constant output voltage at an output terminal. In one embodiment, a LDO regulator includes a power transistor, a feedback network, a control circuit, and an adaptive biasing circuit. The power transistor connects in series between the input terminal and the output terminal of the LDO regulator, wherein the power transistor provides a load current to the output terminal while maintaining a selected output voltage. The feedback network is configured to sense the output voltage and generate a feedback voltage indicative of the output voltage. The control circuit is configured to receive the feedback voltage and to control the power transistor to maintain the output voltage at a substantially constant level (i.e., the selected level). Furthermore, the adaptive biasing circuit is configured to sense changes in the load current and alters an operating current of the LDO regulator in response.
In one embodiment, a method for adaptive biasing of a linear voltage regulator improves transient responses and power efficiency of the linear voltage regulator. The method includes biasing the linear voltage regulator at a relatively low operating current for steady-state operation, thus improving power efficiency of the linear voltage regulator. The method also includes detecting transients in a load current. The method further includes increasing the operating current of the linear voltage regulator to a relatively high level during the transients, thereby improving transient responses of the linear voltage regulator.
In one embodiment, the linear voltage regulator is a LDO regulator with a pass element interposed between an input terminal and an output terminal. A control circuit provides a control signal to drive the pass element to control an output current provided to the output terminal, thereby controlling a corresponding output voltage. In one embodiment, the control circuit is initially biased at a relatively low operating current. The control circuit receives a feedback signal from a feedback circuit. The feedback circuit senses the output voltage at the output terminal and develops the feedback voltage indicative thereof.
In one embodiment, the control circuit compares the feedback voltage to a reference voltage to produce the control signal. The level of the control signal changes as a load current changes to provide load regulation (i.e., maintain a substantially constant output voltage under changing load conditions). For example, as the load current (i.e., the current drawn by a load coupled to the output terminal) increases, the output voltage decreases unless the output current increases correspondingly. In one embodiment, the LDO regulator senses an increase in the load current and increases the operating current of the control circuit to a relatively high level temporarily. The relatively high operating current allows the control signal to transition relatively quickly, thereby improving the transient response of the LDO regulator.